In wireless communication systems, antenna diversity plays an important role in increasing the system link robustness. OFDM is used as a modulation technique for transmitting digital data using radio frequency signals (RF). In OFDM, a radio signal is divided into multiple sub-signals that are transmitted simultaneously at different frequencies to a receiver. Each sub-signal travels within its own unique frequency range (sub-channel), which is modulated by the data. OFDM distributes the data over multiple channels, spaced apart at different frequencies.
OFDM modulation is typically performed using a transform such as Fast Fourier Transform (FFT) process wherein bits of data are encoded in the frequency-domain onto sub-channels. As such, in the transmitter, an Inverse FFT (IFFT) is performed on the set of frequency channels to generate a time-domain OFDM symbol for transmission over a communication channel. The IFFT process converts the frequency-domain data for each sub-channel into a block of time-domain samples, which are converted to an analogue modulating signal later on for an RF modulator. In the receiver, the OFDM signals are processed by performing an FFT process on each OFDM symbol to convert the time-domain data into frequency-domain data, and the data is then decoded by examining the phase and amplitude of the sub-channels. Therefore, at the receiver the reverse process of the transmitter is implemented. Further, transmit antenna diversity schemes are used to improve the OFDM system reliability. Such transmission diversity schemes in OFDM systems are encoded in the frequency-domain as described.
OFDM has been selected as the basis for the high speed wireless local area network (WLAN) standards by the IEEE 802.11a standardization group, and is also being considered as the basis for the high throughput WLAN 802.11n standard. A typical transmitter for a conventional OFDM Multiple Input Multiple Output (MIMO) system implementing WLAN 802.11n comprises a channel encoder, a puncturer, a spatial parser, and multiple data stream processing paths. An example system is described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11 11-04-889r1, November 2004 (incorporated herein by reference). Each data stream processing path comprises an interleaver, a bit-to-symbol constellation mapper, an IFFT function, and guard interval (GI) insertion window and an RF modulator.
For parser and interleaver portion of the system, forward error correction (FEC) coded and punctured bits are interleaved across spatial streams and frequency tones. There are two steps to the space-frequency interleaving: spatial stream parsing and frequency interleaving. First, encoded and punctured bits are parsed to multiple spatial streams by a round-robin parser. The parser sends consecutive blocks of bits to different spatial streams in a round-robin fashion starting with the first spatial stream. Second, all encoded bits are interleaved by a separate block interleaver for each spatial stream, with a block size corresponding to the number of bits in a single OFDM symbol. The block interleavers are based on the 802.11a interleaver, with certain modifications to allow for multiple spatial streams and 40 MHz transmissions.
The interleaver is defined by a two-step permutation. The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. The second permutation ensures that coded bits are mapped alternately onto less and more significant bits of the constellation and thereby long runs of low reliability (LSB) bits are avoided. A deinterleaver in a receiver performs the inverse operation, and is also defined by two permutations corresponding to the two interleaver permutations.
Such conventional system provides write in block, one column rotation for multiple antennas transmission, and PAM order rotation within a column. However, because the columns are rotated by only one column, adjacent bits are only 3 and 6 sub-carriers apart for 20 MHz and 40 MHz systems, respectively. As a result, in a correlated channel, the diversity gain is not fully utilized.
Another conventional transmitter design includes a channel encoder, a puncturer, a frequency interleaver, a spatial parser, and two data stream processing paths. Each data stream processing path comprises a bit to symbol constellation mapper, an IFFT function, guard interval insertion window and an RF modulator. The interleaver performs interleaving on two consecutive OFDM symbols before they are parsed onto two different antennas. The relation for the first permutation is:i=Nrow×(k mod Ncolumn)+floor(k/Ncolumn)
where Ncolumn=32, Nrow=2NCBPS/Ncolumn
After the interleaving, the spatial parser parses the interleaved bits in group by a round robin fashion to different spatial streams. The group size equals to the number of bits in one Quadrature Amplitude Modulation (QAM) symbol. For example, for 64 QAM, 6 bits will be parsed into one spatial stream and the next 6 bits will be parsed into another spatial stream. However, such a transmitter is not flexible enough to accommodate different channel coding and modulation schemes on different special streams.